package com.kent.internet;

import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

/**
 * Notice: 多线程下载工具类
 * 这段代码已经实现了多线程下载的核心代码，如果要实现断点下载，则需要额外增加一个配置文件，断点下载工具下载开始时会生成两个文件：一个是与网络资源大小相同的空文件，一个是配置文件，
 * 该配置文件分别记录每个线程已经下载到哪个字节，当网络断开后再次开始下载时，每个线程根据配置文件里记录的位置向后下载即可。
 *
 * @author kent_sun
 * @version 1.0
 * @date 2021/4/29
 * @since 1.0
 */
public class DownUtil {
    // 定义下载资源的路径
    private String path;
    // 指定所下载的文件的保存位置
    private String targetFile;
    // 定义需要使用多少个线程下载资源
    private int threadNum;
    // 定义下载的线程对象
    private DownThread[] threads;
    // 定义下载的文件的总大小
    private int fileSize;

    public DownUtil(String path, String targetFile, int threadNum) {
        this.path = path;
        this.threadNum = threadNum;
        // 初始化threads数组
        threads = new DownThread[threadNum];
        this.targetFile = targetFile;
    }

    public void download() throws Exception {
        var url = new URL(path);
        var conn = (HttpURLConnection) url.openConnection();
        conn.setConnectTimeout(5 * 1000);
        conn.setRequestMethod("GET");
        conn.setRequestProperty(
                "Accept",
                "image/gif, image/jpeg, image/pjpeg,image/pjpeg, "
                        + "application/x-shockwave-flash, application/xaml+xml, "
                        + "application/vnd.ms-xpsdocument, application/x-mbaP, "
                        + "application/x-ms-application, application/vnd.ms-excel, "
                        + "application/vnd.ms-powerpoint, application/msword, */*");
        conn.setRequestProperty("Accept-Language", "zh-CN");
        conn.setRequestProperty("Charset", "UTF-8");
        conn.setRequestProperty("Connection", "Keep-Alive");
        // 得到文件大小
        fileSize = conn.getContentLength();
        conn.disconnect();
        int currentPartSize = fileSize / threadNum + 1;
        var file = new RandomAccessFile(targetFile, "rw");
        // 设置本地文件的大小
        file.setLength(fileSize);
        file.close();
        for (var i = 0; i < threadNum; i++) {
            // 计算每个线程下载的开始位置
            var startPos = i * currentPartSize;
            // 每个线程使用一个RandomAccessFile进行下载
            var currentPart = new RandomAccessFile(targetFile, "rw");
            // 定位该线程的下载位置
            currentPart.seek(startPos
            );
            // 创建下载线程
            threads[i] = new DownThread(startPos, currentPartSize, currentPart);
            // 启动下载线程
            threads[i].start();
        }
    }

    // 获取下载的完成百分比
    public double getCompleteRate() {
        // 统计多个线程已经下载的总大小
        var sumSize = 0;
        for (int i = 0; i < threadNum; i++) {
            sumSize += threads[i].length;
        }
        // 返回已经完成的百分比
        return sumSize * 1.0 / fileSize;
    }

    private class DownThread extends Thread {
        // 当前线程的下载位置
        private int startPos;
        // 定义当前线程负责下载的文件大小
        private int currentPartSize;
        // 当前线程需要下载的文件块
        private RandomAccessFile currentPart;
        // 定义该线程已下载的字节数
        public int length;

        public DownThread(int startPos, int currentPartSize, RandomAccessFile currentPart) {
            this.startPos = startPos;
            this.currentPartSize = currentPartSize;
            this.currentPart = currentPart;
        }

        public void run() {
            try {
                var url = new URL(path);
                var conn = (HttpURLConnection) url.openConnection();
                conn.setConnectTimeout(5 * 1000);
                conn.setRequestMethod("GET");
                conn.setRequestProperty(
                        "Accept",
                        "image/gif, image/jpeg, image/pjpeg,image/pjpeg, "
                                + "application/x-shockwave-flash, application/xaml+xml, "
                                + "application/vnd.ms-xpsdocument, application/x-mbaP, "
                                + "application/x-ms-application, application/vnd.ms-excel, "
                                + "application/vnd.ms-powerpoint, application/msword, */*");
                conn.setRequestProperty("Accept-Language", "zh-CN");
                conn.setRequestProperty("Charset", "UTF-8");
                InputStream inStream = conn.getInputStream();
                // 跳过startPos个字节，表名该线程只下载自己负责的那部分文件
                inStream.skip(this.startPos);
                var buffer = new byte[1024];
                var hasRead = 0;
                // 读取网络数据，并写入本地文件
                while (length < currentPartSize && (hasRead = inStream.read(buffer)) != -1) {
                    currentPart.write(buffer, 0, hasRead);
                    // 累计该线程下载的总大小
                    length += hasRead;
                }
                currentPart.close();
                inStream.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}
